Protein sequencing method and reagents

ABSTRACT

The invention describes methods and reagents useful for sequencing polypeptide molecules. The method comprises affixing a polypeptide to a substrate and contacting the polypeptide with a plurality of probes. Each probe selectively binds to an N-terminal amino acid or an N-terminal amino acid derivative. Probes bound to the polypeptide molecule are then identified before cleaving the N-terminal amino acid or N-terminal amino acid derivative of the polypeptide. Also provided are methods for the sequencing a plurality of polypeptide molecules in a sample and probes specific for N-terminal amino acids or N-terminal amino acid derivatives.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional application no.61/245,875 titled SINGLE MOLECULE PROTEIN SEQUENCING METHOD filed onSep. 25, 2009, the contents which of are herein incorporated byreference.

NON-PUBLICATION REQUEST

A non-publication request has been submitted with this application uponfiling. This application is not to be published under 35 U.S.C. 122(b)

INCORPORATION OF SEQUENCE LISTING

A computer readable form of the Sequence Listing“13795-15_Sequence_Listing.txt” (4,636 bytes), submitted via EFS-WEB andcreated on Sep. 27, 2010, is herein incorporated by reference.

FIELD OF THE INVENTION

This invention relates to the field of protein sequencing. Morespecifically, the invention relates to methods, assays and reagents forsequencing protein or polypeptide molecules as well as to methods andassays for the parallel sequencing of proteins or polypeptides.

BACKGROUND OF THE INVENTION

Proteins mediate the biological activity, and function, of virtuallyevery biological process in cells, while misexpression is associatedwith various human diseases. The identification and quantification ofproteins present in biological samples is therefore a fundamentalproblem applicable to most biomedical research studies, and acornerstone of the emerging field of Proteomics.

Protein sequencing has traditionally relied on the sequential detectionof individually cleaved N-terminal amino acids from a population ofidentical polypeptide molecules using Edman degradation chemistry andthe detection and identification of the different amino acid Edmanderivatives using techniques such as differential HPLC retention and UVabsorption. More recently, mass spectrometry has been used to sequenceand/or identify proteins or polypeptides with increased speed, accuracyand sensitivity. These methods are generally low-throughput,computationally demanding and require the use of expensive equipment.However, even the most sensitive mass spectrometers require relativelylarge amounts of sample, with current limits of detection on the orderof 10⁸ molecules (equivalent to nanogram or femtomole levels) and arenot able to exhaustively sequence complex mixtures of proteins due toion-ion interference, preferential (biased) detection of certainmolecules, limited dynamic range and general under-sampling.

While dramatic improvements have been made in the past couple of yearswith respect to the speed, comprehensiveness and availability ofhigh-throughput massively parallel DNA sequencing platforms capable ofsequencing large numbers of different nucleic acid moleculessimultaneously, advances in mass spectrometer performance have beenincremental. Relatively little progress has been made towards thedevelopment of “next generation” platforms for global protein sequencingat the individual single molecule level. Furthermore, the relativecomplexity of protein mixtures such as blood, tissue or cell extracts,as well as the lack of PCR-based amplification or properties such asduplex formation and base-pairing, have hampered the development ofsingle-molecule protein sequencing such as those described forpolynucleotides (Harris et aL Science 4 Apr. 2008: Vol. 320. no. 5872).

Accordingly, there remains a need for novel methods and assays forsequencing single polypeptide molecules and for methods and assays ableto perform the simultaneous parallel sequencing of large-numbers ofpolypeptides present in one or more samples.

SUMMARY OF THE INVENTION

In a broad aspect there are provided methods for sequencing polypeptideswherein the polypeptides are contacted with probes that selectively bindto N-terminal amino acid residues. In another broad aspect, there areprovided probes that selectively bind to N-terminal amino acid residues.In one embodiment the methods and reagents are useful for sequencing asingle polypeptide molecule, multiple molecules of a single polypeptide,or for the parallel sequencing of a plurality of different polypeptides.The described methods and reagents are also useful for the massivelyparallel sequencing of mixtures of proteins, such as for the analysis ofsingle cells or biological or environmental samples. In a furtheraspect, the methods are useful for the both the qualitative (i.e.determining the sequence identity) and quantitative (i.e. determiningthe abundance) analysis of protein expression in one or more samples.

Accordingly, in one embodiment, there is provided a method of sequencinga polypeptide comprising affixing the polypeptide to a substrate andcontacting the polypeptide with a plurality of probes where each probeselectively binds to an N-terminal amino acid or a N-terminal amino acidderivative. The probe bound to the polypeptide is then detected therebyidentifying the N-terminal amino acid of the polypeptide. In oneembodiment, the N-terminal amino acid or N-terminal amino acidderivative of the polypeptide is cleaved, and the steps of contactingthe polypeptide with a plurality of probes, detecting the probe bound tothe polypeptide, and cleaving the N-terminal amino acid of thepolypeptide are repeated to determine the sequence of at least a portionof the polypeptide. In some embodiments, rinse or wash steps areincluded before or after each step of the method. Optionally, thepolypeptide is a single polypeptide molecule.

In one embodiment, one or more polypeptides are affixed to a substrateand contacted with a plurality of probes before washing the substrate toremove any non-specifically bound probes.

In one embodiment, the N-terminal amino acid of a polypeptide affixed tothe substrate is derivatized prior to contacting the polypeptide withthe plurality of probes. For example, in one embodiment the N-terminalamino acid is derivatized with an Edman reagent such as phenylisothiocyanate (PITC).

In one embodiment, the polypeptide is affixed to the substrate through aC′-terminal carboxyl group or a side chain functional group of thepolypeptide. In some embodiments the polypeptide is covalently ornon-covalently affixed to the substrate.

In one embodiment, the substrate is optically transparent. For example,the substrate is optionally a glass slide or silicon wafer. Optionally,the substrate is embedded in a microfluidic device.

In one embodiment, the substrate comprises a plurality of spatiallyresolved attachment points. In some embodiments, the attachment pointsinclude a molecular linker, such as a polyethylene glycol (PEG) moiety.In some embodiments, the polypeptide is affixed to the substrate througha spatially resolved attachment point.

In one embodiment, the polypeptide affixed to the substrate is contactedwith a plurality of probes. In one embodiment, the plurality of probesincludes one or more probes that selectively bind to one of 20 naturalproteinogenic amino acids, one or more probes that selectively bind to apost-translationally modified amino acid, or one or more probes thatselectively bind to an amino acid derivative or modified amino acidderivative. In one embodiment, the probes selectively bind a N-terminalamino acid or a N-terminal amino acid derivative. The 20 naturalproteinogenic amino acids include those amino acids commonly found inproteins and coded for by the standard genetic code. In someembodiments, the amino acid derivative is an Edman reagent derivativesuch as a phenylthiocarbamyl (PTC) derivative.

In some embodiments, the probe comprises an affinity capture reagent andone or more detectable labels. Optionally, the affinity capture reagentis a synthetic or natural antibody. In some embodiments, the affinitycapture reagent is an aptamer. In one embodiment, the affinity capturereagent is a polypeptide, such as a modified member of the CIpS familyof adaptor proteins. Optionally, the probe comprises a variant of a E.Coil CIpS binding polypeptide, wherein the variant has at least 80%sequence identity to the polypeptide set forth in SEQ ID NO: 1. In oneembodiment, the probe comprises a polypeptide with at least 80% sequenceidentity to the polypeptide set forth in SEQ ID NO: 2 and is selectivefor N-terminal tryptophan residues.

In one embodiment, the detectable label is optically detectable. In someembodiments, the detectable label comprises a fluorescently moiety, acolor-coded nanoparticle, a quantum dot or any combination thereof. Inone embodiment the label comprises a polystyrene dye encompassing a coredye molecule such as a FluoSphere™.

In one embodiment, the probe bound to the polypeptide affixed to thesubstrate is detected by directly or indirectly detecting the detectablelabel. In some embodiments, the probe is detected using an opticaldetection system. Optionally, the optical detection system comprises aCCD camera and/or a rastering laser/scanner. In some embodiments, theoptical detection system has single-photon resolution.

In one embodiment of the method described herein, the N-terminal aminoacid of the polypeptide affixed to the substrate is cleaved. In oneembodiment, the N-terminal amino acid or N-terminal amino acidderivative is cleaved using Edman degradation. For example in oneembodiment, the Edman degradation proceeds through the addition ofphenylisothiocyanate under alkaline conditions to form a cyclicalphenylthiocarbamoyl derivative followed by cleavage of the N-terminalamino acid under acidic conditions. In one embodiment, the N-terminalamino acid is derivatized at a pH of between 8 and 10, and the step ofcleaving the N-terminal amino acid occurs at a pH of between 2 and 6.

In one embodiment, the polypeptide to be sequenced is a partiallydigested or completely digested protein. In a further embodiment, asample comprising a plurality of polypeptides is treated with anendopeptidase in order to partially or completely digest thepolypeptides contained in the sample. In one embodiment, the polypeptideor sample is digested prior to being affixed to the substrate.

In another aspect, there is provided a method of sequencing a pluralityof polypeptide molecules in a sample comprising affixing the polypeptidemolecules in the sample to a plurality of spatially resolved attachmentpoints on a substrate and contacting the polypeptides with a pluralityof probes, wherein the probes selectively bind to an N-terminal aminoacid or a N-terminal amino acid derivative. In one embodiment, for eachpolypeptide molecule that is spatially resolved and affixed to thesubstrate, the probe bound to each polypeptide is identified. In afurther embodiment, the N-terminal amino acid or N-terminal amino acidderivative of each of the polypeptides is cleaved. In one embodiment,the steps of contacting the plurality of peptides with a plurality ofprobes, identifying the probes bound to the polypeptides and cleavingthe N-terminal amino acid of the polypeptide are repeated in order todetermine the sequence of at least a portion of each polypeptidemolecule that is spatially resolved and affixed to the substrate.

In one embodiment, the N-terminal amino acid residues of thepolypeptides affixed to the substrate are derivatized prior tocontacting the polypeptide with the plurality of probes. For example theN-terminal amino acid residues may be derivatized with an Edman reagent,such as PITC.

In some embodiments, rinse or wash steps are included before or aftereach step of the methods described herein.

In some embodiments, the sample comprises a cell extract or tissueextract. In one embodiment, the sample comprises a single cell. In someembodiments, the sample is a biological fluid such as blood, plasma. Thesample may also comprise an environmental sample such as a soil sampleor other biological material.

In some embodiments, the sequence information generated using themethods described herein is used to search a reference sequencedatabase. For example, in one embodiment, a sequence database issearched using the complete or partial sequence of a polypeptide inorder to determine the identity of the polypeptide.

In another embodiment, the methods described herein are useful for thequantitative analysis of polypeptide in a sample. For example, thenumber of instances of a particular polypeptide in a sample can bedetermined by comparing the sequence or partial sequence of eachpolypeptide, grouping similar polypeptide sequences and counting thenumber of instances of each similar polypeptide sequence.

In one aspect, the description provides probes that selectively bind toan N-terminal amino acid or N-terminal amino acid derivative of apolypeptide. In one embodiment, the probe comprises an antibody orantibody fragment. In another embodiment, the probe comprises anaffinity capture binding reagent. In some embodiments, the affinitycapture binding reagent is an aptamer. In one embodiment, the probesfurther comprise a detectable label such as a fluorescent moiety.

In one embodiment, there is provided a probe comprising a variant of apolypeptide wherein the variant binds to an N-terminal amino acid or aN-terminal amino acid derivative with a different selectivity than thepolypeptide. In one embodiment, probe comprises a variant CIpSpolypeptide, and the variant polypeptide binds to an N-terminal aminoacid with a different selectivity than the CIpS polypeptide. Optionally,the CIpS polypeptide is a truncated E. coli CIpS polypeptide comprisingthe following sequence:

(SEQ ID NO: 1) KPPSMYKVILVNDDYTPMEFVIDVLQKFFSYDVERATQLMLAVHYQGKAICGVFTAEVAETKVAMVNKYARENEHPLLCTLEKA

The inventors have determined that variant CIpS polypeptides withmutations at specific positions in the sequence set forth in SEQ ID NO:1 are useful for selectively binding specific N-terminal amino acids andfor the sequencing methods described herein. In one embodiment, thevariant polypeptides show selectivity for N-terminal amino acidscompared to the parent polypeptide used to generate the variant, such aswildtype CIpS. In one embodiment, the probes show affinity for differentN-terminal amino acids compared to wildtype CIpS. In one embodiment, theprobes comprise variant CIpS polypeptides with one or moresubstitutions, insertion, deletions or additions at residues thatcorrespond to residues 12, 13, 14, 16, 17, 18, 21, 40, 43, 44, 76 and 77as set forth in SEQ ID NO: 1. In one embodiment, the probes comprisevariant CIpS polypeptides with one or more mutations at ligandspecificity pocket residues selected from residues that correspond topositions 17, 18, 21, 40, 43, 76 and 77 as set forth in SEQ ID NO: 1. Inone embodiment, the probes comprise variant CIpS polypeptides with oneor more mutations at alpha amino binding residues selected from residuesthat correspond to positions 12, 13, 14, 16, 17, 44, and 76 as set forthin SEQ ID NO: 1. In one embodiment, the probe comprises a variant of SEQID NO: 1, wherein residue positions 21 and 40 are cystein.

In one embodiment, there is provided a N-terminal amino acid probeselective for tryptophan that comprises the following polypeptidesequence (substitutions relative to SEQ ID NO: 1 shown in underline):

(SEQ ID NO: 2) KPPSMYKVILVNDDYTPAEFCIDVLQKFFSYDVERATQLCLAAHYQGKAICGVFTAEVAETKVAMVNKYARENEHAALCTLEKA

In one embodiment, the probe comprises a variant CIpS polypeptide withat least 70%, 80%, 90%, or 95% sequence identity to SEQ ID NO: 1 or toSEQ ID NO: 2.

In another aspect, the probes are directly or indirectly labeled with adetectable label. In one embodiment, the probes are indirectly labeledthrough conjugation of a glutathione transferase (GST) domain andglutathione. In one embodiment, the probes are directly labeled throughcovalent attachment of the detectable label and the probe. In oneembodiment, the detectable label is a fluorescent label such as aFluoSphere™.

The one aspect, there is also provided a cloned DNA sequence encoding apolypeptide that selectively binds an N-terminal amino acid enzyme. Inone embodiment, the cloned DNA sequence encodes a polypeptide with thesequence set forth in SEQ ID NO: 1 or SED ID NO: 2.

In one aspect, there is provided a method for producing polypeptidesthat selectively bind N-terminal amino acids. In one embodiment, themethod comprises generating a plurality of variant polypeptides,expressing the polypeptides using phage display and selecting forvariants that selectively bind to one or more N-terminal amino acids. Inone embodiment, the variant polypeptides comprise variant CIpSpolypeptides. In one embodiment, the variants are selected undercompetitive conditions. In one embodiment, the method producespolypeptides that selectively bind a target N-terminal amino acid andthe competitive conditions include the presence of peptides withN-terminal amino acids others than the target N-terminal amino acid.

Other features and advantages of the present invention will becomeapparent from the following detailed description. It should beunderstood, however, that the detailed description and the specificexamples while indicating preferred embodiments of the invention aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a schematic showing one embodiment of a method for sequencingpolypeptides as described herein.

FIG. 2 illustrates one embodiment of N-terminal Edman degradation of asingle polypeptide.

FIG. 3 illustrates one embodiment of affixing a polypeptide to asubstrate using a polyethylene glycol (PEG) chemical linker.

FIG. 4 shows sequence alignment of selected bacterial CIpS proteins (SEQID NOS: 6-15) together with the type 2 binding region (UBR box) from thehomologous eukaryotic UBR1 and UBR2 proteins (SEQ ID NOS: 16-21) (fromSchuenemann et al. EMBO Reports, Vol. 10, No. 5, 2009).

FIG. 5 shows directed evolution of engineered protein domains for use inN-terminal amino acid probes using phage display. Large combinatoriallibraries, generated by targeted Kunkel mutagenesis or by chemical genesynthesis, of domain variants (˜10¹⁰ unique individuals) can bedisplayed as fusions to bacteriophage coat proteins, such that thesequence of the displayed protein on a single phage particle is encodedby the genome packaged within. This physical connection betweenphenotype and genotype makes it possible to select a displayed variantbased on its binding properties, and then to obtain its sequence fromthe encapsulated phage genome. Selections are performed by immobilizingthe ligand of interest to a solid support, allowing the library ofvariants an opportunity to bind, and then washing away all phage thatfail to bind. The phage that do bind the ligand are retained, andinfected into bacteria in order to replicate. The same selectionprocedure can then be repeated with the new subpopulation of phage.After repeated rounds of selection, binders will dominate, at whichpoint individual clones can be isolated, sequenced and subcloned forexpression as recombinant proteins and further biochemicalcharacterization.

FIG. 6A shows the CIpS domain displayed on page with an N-terminal FLAGtag. FIG. 6B shows the results of phage binding to immobilized peptidesor control peptides assessed using an enzyme linked immunosorbent assay(ELISA). Selected peptide ligands during were synthesized withC-terminal biotin-lysine residues. The biotinylated peptides wereimmobilized on neutravidin-coated wells of a microtiter plate by passiveadsorption, alongside additional control proteins. CIpS from E. colibinds two of the Leu-peptides specifically, without cross reactivity toany of the negative controls (bovine serum albumin, BSA; streptavidin,SA; neutravidin, NA). The phage-displayed Fabs are also FLAGtagged andserve as negative controls for non-specific binding to the immobilizedpeptides. FIG. 6C shows the recombinant phage coat protein comprisingthe ST2 secretion signal, the flag tab, the truncated wild type CIpSpolypeptide and gene 3 (SEQ ID NO: 3) used in the phage displayexperiments as set out in Example 4.

FIG. 7 shows the relative positions of the key residues hard randomizedfor the CIpS (SEQ ID NO: 22) phage display libraries for sidechainspecificity (FIG. 7A) and alpha-amino binding (FIG. 7B). FIG. 7A:Sidechain specificity is dictated by the shape and chemical compositionof the hydrophobic pocket, mediated by sidechains of the highlightedresidues. Highlighted residues were chosen for randomization based onanalysis of the CIpS structure and available data; their sidechainscontribute to the hydrophobic pocket either directly or by influencingthe orientation of direct contacts (i.e. two proline residues). FIG. 7B:alpha-amino recognition depends on a network of hydrogen bonds mediatedby the residues shown at positions 34, 35, 36, 38 and 66, through acombination of sidechain and main chain interactions. The two prolineresidues shown at positions 39 and 98 influence the orientation of thesecritical residues as well.

FIG. 8 shows that CIpS (SEQ ID NO: 22) is structurally tolerant ofdiverse residues at key specificity mediating positions.Protease-resistant, likely well-folded variants, were selected from eachlibrary using an immobilized anti-FLAG antibody. The distribution ofresidues at each randomized position in each set of structure-selectedCIpS variants is shown as a frequency logo. FIG. 8A: Positions in thesidechain specificity pocket are surprisingly structurally tolerant to awide variety of substitutions. The exclusively hydrophobic residues inthe wildtype domain can also be substituted with polar or chargedresidues. FIG. 8B: Alpha-amino recognition residues are also quitestructurally tolerant to substitution. The set of structurally-toleratedresidues at each of these positions informs subsequent library design,and provides a useful basis for comparison for residue frequenciesobserved in function-selected variants.

FIG. 9 shows an indirect labeling methodology for optical detection ofrecombinant probes. FIG. 9A: FluoSpheres (FS) functionalized withglutathione (GSH) bind the GST domain of the recombinant CIpS proteins.Each population of CIpS protein can be tagged with its own colour labelin a simple one-step process that requires no chemical modification.FIG. 9B: Bioconjugation scheme to adsorb GSH onto FluoSphere surface ina manner that does not inhibit GST binding. FS surface, initiallycontaining carboxylic acid, is first reacted with ethylene diamine toprovide amine groups on the FluoSphere surface, which are further usedto react with Sulfo-LC-SPDP. Reaction with Sulfo-LCSPDP leaves a thiolreactive end, which reacts with GSH upon mixing.

FIG. 10 shows a direct chemical labeling methodology for opticaldetection of recombinant probes by tagging CIpS proteins or otheraffinity capture reagents comprising polypeptides with fluorescentlabels using an EDC linker. Amines of CIpS proteins are covalentlyconjugated to carboxylic acids on FluoSpheres through EDC-catalyzedreaction. This labeling approach does not require GSH functionalizationof FluoSpheres, but the location of the reacting amine cannot becontrolled.

FIG. 11 shows the optical detection of preferential ligand binding bylabeled CIpS probes. FIG. 11A: Immobilized peptide binding signalobtained with immobilized peptides probed using CIpS covalently attachedto fluospheres using EDC linker. FIG. 11B: Binding signal produced withFluospheres attached to probe using indirect non-covalent GST-GSHinteraction. Both labeled probes show a fluorescent signal indicatingpreferential binding to the target Leucine N-terminal amino acid.

FIG. 12 shows the sequence of CIpS variant 1 that binds N-terminaltryptophan residues selectively. FIG. 12 A: Sequence of the wildtypeCIpS domain from E. coli that recognizes N-terminal F, L, Y and Wresidues (SEQ ID NO: 1) (top) and the engineered variant that bindsN-terminal W residues selectively (SEQ ID NO: 2) (bottom) with keyresidues highlighted. FIG. 12B: structure of wildtype CIpS in complexwith Lpep (PDB: 2W9R) with diversified residues in the displayed libraryshown as highlighted. FIG. 12C: Key residues in the novel CIpS variantW1 are indicated on the wildtype structure.

FIG. 13 shows results from competitive phage ELISA demonstrating thatonly Wpep is an effective competitor for binding CIpS Variant 1. Errorbars represent 95% confidence intervals (n=2).

DETAILED DESCRIPTION OF THE INVENTION

The present description provides methods, assays and reagents useful forsequencing proteins. In one aspect, the methods are useful forsequencing single polypeptide molecules or multiple molecules of asingle polypeptide. In one aspect, the methods and reagents are usefulfor determining the N-terminal amino acid of a polypeptide. In oneaspect, the methods are useful for the simultaneous sequencing of aplurality of single polypeptide molecules, such as for the basis ofmassively parallel sequencing techniques. Accordingly samples comprisinga mixture of different proteins can be assayed according to the methodsdescribed herein to generate sequence information regarding individualprotein molecules in the sample. In a further aspect, the methods areuseful for protein expression profiling in complex samples. For example,the methods are useful for generating both quantitative (frequency) andqualitative (sequence) data for proteins contained in a sample.

In a further aspect, the description provides reagents such asN-terminal amino acid affinity capture reagents and probes comprisingN-terminal amino acid affinity capture reagents suitable for practicingthe methods described herein. In one embodiment, probes are variant CIpSpolypeptides generated as set forth in Example 4. In one embodiment, theprobes comprise readily detectable labels, such as fluorescent dyes.

The inventors have determined that probes specific for an N-terminalamino acid of a polypeptide, or specific for a derivative of aN-terminal amino acid of a polypeptide, can be used to generate sequenceinformation by sequentially identifying and then cleaving the N-terminalamino acids of a polypeptide. The inventors have also determined that byfirst affixing the polypeptide molecule to a substrate, it is possibleto determine the sequence of that immobilized polypeptide by iterativelydetecting which probes are bound to the polypeptide at that samelocation on the substrate. Each probe may contain one or more detectablelabel(s) that facilitates the identification of specific probes.

Accordingly, in one embodiment there is provided a method of sequencinga polypeptide comprising:

-   -   a) affixing the polypeptide to a substrate;    -   b) contacting the polypeptide with a plurality of probes,        wherein each probe selectively binds to an N-terminal amino acid        or a N-terminal amino acid derivative;    -   c) detecting the probe bound to the polypeptide molecule,        thereby identifying the N-terminal amino acid of the        polypeptide;    -   d) cleaving the N-terminal amino acid or N-terminal amino acid        derivative of the polypeptide; and    -   e) repeating steps (b) to (d) to determine the sequence of at        least a portion of the polypeptide.

Optionally, step a) comprises affixing a plurality of polypeptides withthe same sequence to the substrate and step c) comprises detecting aplurality of probes. In one embodiment, the polypeptide comprises asingle polypeptide molecule and step c) comprises detecting a singleprobe bound to the polypeptide molecule.

In another embodiment, there is provided a method of sequencing aplurality of polypeptide molecules in a sample comprising:

-   -   a) affixing the polypeptide molecules in the sample to a        plurality of spatially resolved attachment points on a        substrate;    -   b) contacting the polypeptides with a plurality of probes,        wherein each probe selectively binds to an N-terminal amino acid        or a N-terminal amino acid derivative;    -   c) for a plurality of polypeptide molecules that are spatially        resolved and affixed to the substrate, detecting the probe bound        to each polypeptide;    -   d) cleaving the N-terminal amino acid or N-terminal amino acid        derivative of each of the polypeptides; and    -   e) repeating steps b) to d) to determine the sequence of at        least a portion of one or more of the plurality of polypeptide        molecules that are spatially resolved and affixed to the        substrate.

As used herein, “polypeptide” refers to two or more amino acids linkedtogether by a peptide bond. The term “polypeptide” includes proteinsthat have a C-terminal end and an N-terminal end as generally known inthe art and may be synthetic in origin or naturally occurring. As usedherein “at least a portion of the polypeptide” refers to 2 or more aminoacids of the polypeptide. Optionally, a portion of the polypeptideincludes at least: 5, 10, 20, 30 or 50 amino acids, either consecutiveor with gaps, of the complete amino acid sequence of the polypeptide, orthe full amino acid sequence of the polypeptide.

As used herein the phrase “selectively binds to an N-terminal amino acidor a N-terminal amino acid derivative” refers to a probe with a greateraffinity for one or more target N-terminal amino acids or for one ormore N-terminal amino acid derivatives compared to other N-terminalamino acids or N-terminal amino acid derivatives. A probe selectivelybinds a target N-terminal amino acid or N-terminal amino acidderivatives if there is a detectable relative increase in the binding ofthe probe to a target N-terminal amino acid or N-terminal amino acid.For example, as shown in Example 4 and Table 2 the CIpS Variant 1polypeptide selectively binds tryptophan N-terminal amino acids and theCIpS Variant 2 selectively binds arginine and leucine N-terminal aminoacids. Optionally, a probe that is selective is specific for a singleN-terminal amino acid or N-terminal amino acid derivative. Optionally, aprobe that is selective for a N-terminal amino acid or N-terminal aminoacid derivative has at least 25%, 50%, 100%, 200%, or greater than 200%more affinity for a target N-terminal amino acid or N-terminal aminoacid derivative compared to a non-target N-terminal amino acid orN-terminal amino acid derivative. In one embodiment, the probesselectively bind the N-terminal amino acid or N-terminal amino acidderivative with an IC50 of at least 10 micromolar.

The phrase “N-terminal amino acid” refers to an amino acid that has afree amine group and is only linked to one other amino acid by a peptidebond in the polypeptide. The phrase “N-terminal amino acid derivative”refers to an N-terminal amino acid residue that has been chemicallymodified, for example by an Edman reagent or other chemical in vitro orinside a cell via a natural post-translational modification (e.g.phosphorylation) mechanism.

As used herein, “sequencing a polypeptide” refers to determining theamino acid sequence of a polypeptide. The term also refers todetermining the sequence of a segment of a polypeptide or determiningpartial sequence information for a polypeptide.

As used herein, “affixed” refers to a connection between a polypeptideand a substrate such that at least a portion of the polypeptide and thesubstrate are held in physical proximity. The term “affixed” encompassesboth an indirect or direct connection and may be reversible orirreversible, for example the connection is optionally a covalent bondor a non-covalent bond.

As used herein “the cleaving the N-terminal amino acid or N-terminalamino acid derivative of the polypeptide” refers to a chemical reactionwhereby the N-terminal amino acid or N-terminal amino acid derivative isremoved from the polypeptide while the remainder of the polypeptideremains affixed to the substrate.

As used herein the term “sample” includes any material that contains oneor more polypeptides. Samples may be biological samples, such asbiopsies, blood, plasma, organs, organelles, cell extracts, secretions,urine or mucous, tissue extracts and other biological samples of fluidsboth natural or synthetic in origin. The term sample also includessingle cells. The sample may be derived from a cell, tissue, organism orindividual that has been exposed to an analyte (such as a drug), orsubject to an environmental condition, genetic perturbation, orcombination thereof. The organisms or individuals may include, but arenot limited to, mammals such as humans or small animals (rats and micefor example).

As used herein, the term “spatially resolved” refers to an arrangementof two or more polypeptides on a substrate wherein chemical or physicalevents occurring at one polypeptide can be distinguished from thoseoccurring at the second polypeptide. For example, two polypeptidesaffixed on a substrate are spatially resolved if a signal from adetectable label bound to one of the polypeptides can be unambiguouslyassigned to one of the polypeptides at a specific location on thesubstrate.

Substrate Materials

In one embodiment, polypeptides to be sequenced are affixed to asubstrate. In some embodiments, the substrate is made of a material suchas glass, quartz, silica, plastics, metals, composites, or combinationsthereof. In one embodiment, the substrate is a flat planar surface. Inanother embodiment, the substrate is 3-dimensional and exhibits surfacefeatures. In some embodiments, the substrate is a chemically derivatizedglass slide or silica wafer.

In one embodiment, the substrate is made from material that does notsubstantially affect the sequencing reagents and assays describedherein. In one embodiment, the substrate is resistant to the basic andacidic pH, chemicals and buffers used for Edman degradation. Thesubstrate may also be covered with a coating. In some embodiments, thecoating is resistant to the chemical reactions and conditions used inEdman degradation. In some embodiments, the coating provides attachmentpoints for affixing polypeptides to the substrate, and/or repellingnon-specific probe adsorption.

In some embodiments, the surface of the substrate is resistant to thenon-specific adhering of polypeptides or debris, so as to minimizebackground signals when detecting the probes.

In one embodiment, the substrate made of a material that is opticallytransparent. As used herein, “optically transparent” refers to amaterial that allows light to pass through the material. In oneembodiment, the substrate is minimally-or non-autofluorescent.

Optionally, the substrate is embedded in a microfluidic device. In oneembodiment, the microfluidic device is able to direct the reagentsdescribed herein to the surface of the substrate. In another embodiment,multiple substrates are embedded in a microfluidic device.

Affixing Polypeptides to the Substrate

In one embodiment, the polypeptides are affixed to the substrate.Preferably, the polypeptides are affixed to the substrate such that theN-terminal end of the polypeptide is free to allow the binding ofN-terminal amino acid probes. Accordingly, in some embodiments thepolypeptide is affixed to the substrate through the C-terminal end ofthe polypeptide, the C-terminal carboxylic acid group or a side chainfunction group of the polypeptide. In some embodiments, the substratecontains one or more attachment points that permit a polypeptide to beaffixed to the substrate.

In some embodiments, the polypeptide is affixed through a covalent bondto the substrate. For example, the surface of the substrate may containa polyethylene glycol (PEG) or carbohydrate-based coating and thepolypeptides are affixed to the substrate via an N-hydroxysuccinimide(NHS) ester PEG linker.

FIG. 3 shows one example of a polypeptide linked to a substrate througha PEG-based molecular linker tethered to the surface. A number ofdifferent chemistries for attaching linkers and polypeptides to asubstrate are known in the art, for example by the use of specializedcoatings that include aldehydesilane, epoxysilane or other controlledreactive moieties. In one embodiment, the substrate is glass coated withSilane or related reagent and the polypeptide is affixed to thesubstrate through a Schiff's base linkage through an exposed lysineresidue.

In some embodiments the polypeptide is affixed non-covalently to thesubstrate. For example, in one embodiment the C-terminal end of thepolypeptide is conjugated with biotin and the substrate comprises avidinor related molecules. As shown in Example 4, peptides may bebiotinylated and readily attached to a neutravidin substrate andsubsequently contacted with N-terminal amino acid probes that bind tothe peptides. In another embodiment, the C-terminal end of a polypeptideis conjugated to an antigen that binds to an antibody or relatedaffinity capture reagent on the surface of the substrate. Additionalcoupling agents suitable for affixing a polypeptide to a substrate havebeen described in the art (See for example, Athena L. Guo and X. Y. Zhu.The Critical Role of Surface Chemistry In Protein Microarrays inFunctional Protein Microarrays in Drug Discovery)

Pre-Treatment of the Polypeptide

In one embodiment, the polypeptide or a sample containing one or morepolypeptides is pre-treated prior to being affixed on the substrate. Forexample, the sample may be concentrated and purified to removecontaminating materials, such as by HPLC or nuclease treatment. Inanother embodiment, the sample may be treated with a protease orpeptidase, which cleaves the polypeptide at a specific residue. In oneembodiment, the polypeptides are tryptic peptides that are coupled viaC-terminal lysine (or arginine) residues, such that the last residue ofthe polypeptide sequenced is inferred to be K (lysine) or R (arginine).

Washing the Substrate

Optionally, after the polypeptides are affixed to the substrate, thesubstrate is washed in order to remove any debris such as unbound probe,nucleic acids or other molecules that are not affixed to the substratethat contribute to generating signal noise that interference with thesequencing of specific polypeptide molecules.

In another embodiment, the substrate is treated in order to chemicallyblock any unused attachment points on the surface of the substrate thatcould result in non-specific binding of probes to the substrate.

N-terminal Amino Acid Probes

In one aspect of the description, there are provided probes thatselectively bind to an N-terminal amino acid or a N-terminal amino acidderivative. In one embodiment, probes that selectively bind to anN-terminal amino acid or an N-terminal amino acid derivative are used tosequence a polypeptide. In some embodiments, the probes are detectablewith single molecule sensitivity.

In some embodiments, a probe selectively binds more than onepre-determined N-terminal amino acid. Probes that selectively bind morethan one N-terminal amino acid may also be used to determine partialsequence information for a polypeptide.

In one embodiment, the probes include 20 probes that each selectivelybind to one of the 20 natural proteinogenic amino acids. In anotherembodiment, the probes include 20 probes that each selectively bind to aderivative of one of the 20 natural proteinogenic amino acids. In oneembodiment, the derivatives are phenylthiocarbamyl derivatives. In afurther embodiment, the probes include probes that selectively bind topost-translationally-modified amino acids or their derivatives.

Probes and Affinity Capture Reagents

In one embodiment, the probes comprise an affinity capture reagent.Affinity capture reagents useful for the methods described herein bindto N-terminal amino acids or their derivatives.

In one embodiment, the affinity capture reagent is a natural orsynthetic antibody or antibody fragment, or derivative thereof. Forexample, antibodies that bind to specific amino acid are known in theart and are available commercially from Millipore Corporation(Billerica, Mass.).

In one embodiment, affinity capture reagents and/or analogous N-terminalbinding proteins are engineered using phage display (i.e. experimentallyvia empirical selection) or through rational protein design (i.e.computationally using structural biology concepts/programs, like dockingpredictions to optimize protein/amino acid ligand interface residues toconfer N-terminal residue binding specificity and/or adapt the bindingto Edman or other forms of modified N-terminal amino acid residues.

In one embodiment, the probe comprises a variant of a polypeptidewherein the variant binds to an N-terminal amino acid or a N-terminalamino acid derivative with a different selectivity than the polypeptide.As used herein the term “variant” refers to a polypeptide that has oneor more substitutions, additions or deletions compared to a referencenon-variant polypeptide sequence such as a naturally occurringpolypeptide. For example, SEQ ID NO: 2 is a variant of the E. Coli CIpSpolypeptide as set forth in SEQ ID NO: 1. As used herein, a variantpolypeptide has a “different selectivity” than a polypeptide if has agreater affinity for one or more N-terminal amino acids or N-terminalamino acid derivatives than the non-variant polypeptide, or if itexhibits an binding affinity for one or more N-terminal amino acids or aN-terminal amino acid derivatives not seen in the non-variantpolypeptide.

In one embodiment, the affinity capture reagent comprises a member ofthe UBR box recognition sequence family, or a variant of the UBR boxrecognition sequence family. UBR recognition boxes are described inTasaki et al. (Journal of Biological Chemistry, Vol. 284, No. 3 pp.1884-1895 Jan. 16, 2009). Sequence identity between bacterial CIpSproteins and UBR1 and UBR2 is show in FIG. 4.

In a further embodiment, the affinity capture reagent is a member of theevolutionarily conserved CIpS family of adaptor proteins involved innatural N-terminal protein recognition and binding or a variant thereof.The CIpS family of adaptor proteins in bacteria are described inSchuenemann et al. (Structural basis of N-end rule substrate recognitionin Escherichia coli by the CIpAP adaptor protein CIpS. EMBO reports Vol.10, No. 5, 2009), and, Roman-Hernandez et al. (Molecular basis ofsubstrate selection by the N-end rule adaptor protein CIpS. PNAS Jun. 2,2009 vol. 106 no. 22 p. 8888-8893). In some embodiments, the amino acidresidues corresponding to the CIpS hydrophobic binding pocket identifiedin Schuenemann et al. are modified in order to generate affinity capturereagents with the desired selectivity. In one embodiment, the CIpSsequences shown in FIG. 4 are modified such as Met40 or Met62 aremodified to generate novel affinity capture reagents.

As shown in Examples 1 and 4, CIpS adaptor proteins may be modified inorder to selectively bind specific N-terminal amino acids or N-terminalamino acid derivatives. For example, selective N-terminal amino acidprobes may be generated by creating variants of the E. Coli CIpSpolypeptide as set forth in SEQ ID NO: 1, or of the variant Trp-bindingpolypeptide as set forth in SEQ ID NO: 2. Other CIpS polypeptides, suchas those isolated from different species, may also be modified asdescribed herein to generate N-terminal amino acid binding peptides.

In one embodiment, the variant CIpS polypeptides comprises one or moresubstitutions, deletions or additions at positions that correspond toresidues 12, 13, 14, 16, 17, 18, 21, 40, 43, 44, 76 and 77 as set forthin SEQ ID NO: 1. In one embodiment, the N-terminal amino acid probecomprises a variant CIpS polypeptide with cystein residues at positions21 and 40 as set forth in SEQ ID NO: 1. Variant CIpS polypeptides withcystein residues at positions that correspond to residues 21 and 40 asset forth in SEQ ID NO: 1 are believed to form a disulfide bridge,thereby increasing the stability of the variant polypeptide.

As used herein a residue position in one sequence “corresponds to” aresidue position in another polypeptide sequence if it exists in anequivalent position in the polypeptide sequence, as indicated by primarysequence homology, tertiary structural homology (as shown by, e.g.,crystal structure or computer modeling) or functional equivalence. Inone embodiment, sequence alignment between two or more CIpS sequences isused to determine sequence corresponding residues. For example, CIpSpolypeptide sequences isolated from different species can be aligned toidentify residues that correspond to the same position as shown in FIG.4.

As shown in Example 4 and Table 2, CIpS Variant 1 (SEQ ID NO: 2) gave astrong signal and selectively binds polypeptides with the N-terminalamino acid tryptophan. In one embodiment the probes comprise variants ofthe polypeptide sequence set forth in SEQ ID NO: 2. In one embodiment,the variants comprise mutations of the amino acid residues that form thespecificity binding pocket or alpha-amino binding residues of the CIpSprotein.

In one embodiment, the probes comprise polypeptides with at least 70%,80%, 90% or 95% sequence identity to SEQ ID 1 or SEQ ID NO: 2. As usedherein, “sequence identity” refers to the similarity of two polypeptidesequences that are aligned so that the highest order match is obtained.Sequence Identity is calculated according to methods known in the art.For example, polypeptide sequence identity may be calculated usingcomputer programs to determine identity between two sequences.Representative computer programs include, but are not limited to, theGCG program package, FASTA, BLASTP, and TBLASTN (see, e.g., D. W. Mount,2001, Bioinformatics: Sequence and Genome Analysis, Cold Spring HarborLaboratory Press, Cold Spring Harbor, N.Y.). The BLASTP and TBLASTNprograms are publicly available from NCBI and other sources. The SmithWaterman algorithm may also be used to determine percentage sequenceidentity.

Exemplary parameters for determining polypeptide sequence identityinclude the following: 1) algorithm from Needleman and Wunsch (J. Mol.Biol., 48:443-453 (1970)); 2) BLOSSUM62 comparison matrix from Hentikoffand Hentikoff (Proc. Natl. Acad. Sci. U.S.A., 89:10915-10919 (1992)) 3)gap penalty=12; and 4) gap length penalty=4. A program useful with theseparameters is publicly available as the “gap” program (Genetics ComputerGroup, Madison, Wis.). The aforementioned parameters are the defaultparameters for polypeptide comparisons (with no penalty for end gaps).

Alternatively, polypeptide sequence identity can be calculated using thefollowing equation: % sequence identity=(the number of identicalresidues)/(alignment length in amino acid residues)*100. For thiscalculation, alignment length includes internal gaps but does notinclude terminal gaps.

Additional examples of suitable affinity capture reagents include CIpSfamily related members present in non-bacterial species, includinghuman, or synthetic constructs developed using principled computationalmodeling procedures or selected through combinatorial geneticmutagenesis methods.

In another embodiment, affinity capture reagents suitable for use in theprobes described herein are based on structured RNA molecules. In oneembodiment, the affinity capture reagents are aptamers derived bygenerating/screening randomized nucleic acid libraries.

In one aspect, the probes and affinity capture reagents exhibit hightarget selectivity and bind to a limited number N-terminal amino acidsof their derivatives, or preferably only a single N-terminal amino acidor its derivative. In another aspect, the affinity capture reagents arespecific and exhibit a big differential in target to non-targetbinding/affinity. Additional properties of preferred affinity capturereagents include fast “on” (association) kinetic binding rates, slow“off” (dissociation) rates, and limited non-specific absorption. In oneembodiment, the probes and affinity capture reagents exhibit goodstability in solution at different buffers/pHs (e.g. stay folded), along-shelf life, do not require freezing. In one embodiment, the probesand/or affinity capture reagents and can be dried and resolubilizedwithout a significant loss in activity. In one embodiment, the affinitycapture reagents are included in a kit for practicing the methodsdescribed herein. In another embodiment, the affinity capture reagentsare readily synthesized and exhibit a good yield in recombinant orsynthetic form. In another embodiment, the affinity capture reagentsexhibit good tractability in terms of genetic selection and screeningprocedures (e.g. subjected to mutagenesis, protein engineering, or phagedisplay, followed by in vitro binding assay robustness andreproducibility.

Detectable Labels

In another aspect of the description, the probes include detectablelabels. Detectable labels suitable for use with the present inventioninclude, but are not limited to, labels that can be detected as a singlemolecule.

In one embodiment, the probes are detected by contacting the probe witha probe-specific antibody and the probe-specific antibody is thendetected.

In some embodiments, the probes or labels are detected using magnetic orelectrical impulses or signals.

In one embodiment, the labels are optically detectable, such as labelscomprising a fluorescent moiety. Examples of optically detectable labelsinclude, but are not limited to fluorescent dyes including polystyreneshells encompassing core dyes such as FluoSpheres™, Nile Red,fluorescein, rhodamine, derivatized rhodamine dyes, such as TAMRA,phosphor, polymethadine dye, fluorescent phosphoramidite, TEXAS RED,green fluorescent protein, acridine, cyanine, cyanine 5 dye, cyanine 3dye, 5-(2′-aminoethyl)-aminonaphthalene-1-sulfonic acid (EDANS), BODIPY,120 ALEXA or a derivative or modification of any of the foregoing.Additional detectable labels include color-coded nanoparticles, orquantum dots or FluoSpheres™. In one embodiment, the detectable label isresistant to photobleaching while producing lots of signal (such asphotons) at a unique and easily detectable wavelength, with highsignal-to-noise ratio.

One or more detectable labels can be conjugated to the affinity capturereagents described herein using techniques known to a person of skill inthe art. In one embodiment, a specific detectable label (or combinationof labels) is conjugated to a corresponding affinity capture reagentthereby allowing the identification of the affinity capture reagent bymeans of detecting the label(s).

For example, one or more detectable labels can be conjugated to theaffinity capture reagents described herein either directly or indirectlyas shown in Example 5 and FIGS. 9 and 10.

Detecting Probes Bound to the Polypeptides

In still another aspect of the invention, probes bound to a polypeptideaffixed to the substrate are detected, thereby identifying theN-terminal amino acid of the polypeptide. As shown in Example 5, probescomprising a CIpS polypeptide domain can be fluorescently labeled andused to discriminate between N-terminal amino acid residues and generatesequence information.

In one embodiment, the probe is identified by detecting a detectablelabel (or combination of labels) conjugated to the probe. Methodssuitable for detecting the probes described herein therefore depend onthe nature of the detectable label(s) used in the method.

In one embodiment, the probes or labels bound to a polypeptide affixedto a substrate are repeatedly detected at that location using a highresolution rastering laser/scanner across a pre-determined grid, uniqueposition or path on a substrate. These methods are useful for theaccurate and repeated detection of signals at the same spatiallyresolved coordinates during each sequencing cycle of the methodsdescribed herein. In some embodiments, the polypeptides are randomlyaffixed to the substrate and the detection of probes proceeds byrepeatedly scanning the substrate to identify the co-ordinates andidentities of probes bound to polypeptides affixed to the substrate.

In one embodiment, the detecting the probes includes ultrasensitivedetection systems that are able to repeatedly detect signals fromprecisely the same co-ordinates on a substrate, thereby assigning thedetected sequence information to a unique polypeptide molecule affixedat that co-ordinate.

In one embodiment, the probes are detected using an optical detectionsystem. Optical detection systems include a charge-coupled device (CCD),near-field scanning microscopy, far-field confocal microscopy,wide-field epi-illumination, light scattering, dark field microscopy,photoconversion, single and/or multiphoton excitation, spectralwavelength discrimination, fluorophore identification, evanescent waveillumination, total internal reflection fluorescence (TIRF) microscopy,super-resolution fluorescence microscopy, and single-moleculelocalization microscopy. In general, methods involve detection oflaser-activated fluorescence using a microscope equipped with a camera,sometimes referred to as high-efficiency photon detection system.Suitable photon detection systems include, but are not limited to,photodiodes and intensified CCD cameras.

In one embodiment, examples of techniques suitable for single moleculedetection of fluorescent probes include confocal laser (scanning)microscopy, wide-field microscopy, near-field microscopy, fluorescencelifetime imaging microscopy, fluorescence correlation spectroscopy,fluorescence intensity distribution analysis, measuring brightnesschanges induced by quenching/dequenching of fluorescence, orfluorescence energy transfer.

Cleaving the N-terminal Amino Acid

In a further aspect of the description, the N-terminal amino acid of thepolypeptide is cleaved. Cleaving exposes the N-terminus of an adjacentamino acid on the polypeptide, whereby the adjacent amino acid isavailable for reaction with a probe selective for that amino acid.Optionally, the polypeptide is sequentially cleaved until the last aminoacid in the polypeptide (C-terminal amino acid). In some embodiments,the C-terminal amino acid is covalently affixed to the substrate and isnot cleaved from the substrate.

In one embodiment, sequential Edman degradation is used to cleave theN-terminal amino acid of the polypeptide. Edman degradation generallycomprises two steps, a coupling step and a cleaving step. These stepsmay be iteratively repeated, each time removing the exposed N-terminalamino acid residue of a polypeptide. As shown in FIG. 2, in oneembodiment Edman degradation proceeds by way of contacting thepolypeptide with a suitable Edman reagent such as PITC or a PITCanalogue at an elevated pH to form a N-terminal phenylthiocarbamylderivative. Reducing the pH, such by the addition of trifluoroaceticacid results in the cleaving the N-terminal amino acidphenylthiocarbamyl derivative from the polypeptide to form a freeanilinothiozolinone (ATZ) derivative. Optionally, this ATZ derivativemay be detected, or in another embodiment it may then be washed awayfrom the substrate. In one embodiment the pH of the substrate'senvironment in controlled in order to control the reactions governingthe coupling and cleaving steps.

In some embodiments, the N-terminal amino acid is contacted with asuitable Edman reagent such as PITC or a PITC analogue at an elevated pHprior to contacting the affixed polypeptide with a plurality of probesthat selectively bind the N-terminal amino acid derivative. Optionally,the cleaving step comprises reducing the pH in order to cleave theN-terminal amino acid derivative.

In some embodiments, a probe bound to the N-terminal amino acid or itsderivative of a polypeptide is removed prior to cleaving the residue.

In one embodiment, the steps of contacting the polypeptide with aplurality of probes, detecting the probe bound to the polypeptide andcleaving the N-terminal amino acid or N-terminal amino acid derivativeare repeated in order to sequence the polypeptide. Optionally, the stepsare repeated at least 2, 5, 10, 20, 30, 50, or greater than 50 times inorder to sequence part of or the complete polypeptide. Optionally atleast: 5, 10, 20 30 or 50 contiguous or discontiguous amino acidresidues of the amino acid sequence of the polypeptide or the full aminoacid sequence of the polypeptide are determined.

In one embodiment, the method includes washing or rinsing the substratebefore or after any one of the steps of affixing the substrate,contacting the polypeptide with a plurality of probes, detecting theprobe bound to the polypeptide and cleaving the N-terminal amino acid orN-terminal amino acid derivative. Washing or rinsing the substrateremoves waste products such as cleaved N-terminal amino acids, debris orpreviously unused reagents from the substrate that could interfere withthe next step in the sequencing assay.

Parallel Sequencing

The methods described herein allow for the sequencing of very largenumber of polypeptide molecules on a single substrate or on a series ofsubstrates. Accordingly, one aspect of the invention provides forsimultaneously sequencing a plurality of affixed polypeptide moleculesinitially present in a sample. In one embodiment, the sample comprises acell extract or tissue extract. In some embodiments, the methodsdescribed herein may be used to analyze the polypeptides contained in asingle cell. In a further embodiment, the sample may comprise abiological fluid such as blood, urine or mucous. Soil, water or otherenvironmental samples bearing mixed organism communities are alsosuitable for analysis.

In one embodiment of the description, the method includes comparing thesequence of each polypeptide molecule to a reference protein sequencedatabase. In some embodiments, small fragments comprising 10-20 or fewersequenced amino acid residues may be useful for detecting the identityof a polypeptide in a sample.

In one embodiment, the method includes de novo sequencing ofpolypeptides in order to generate sequence information about thepolypeptide. In another embodiment, the method includes determining apartial sequence or an amino acid pattern and then matching the partialsequence or amino acid patterns with reference sequences or patternscontained in a sequence database.

In one embodiment, the method includes using the sequence data generatedby the method as a molecular fingerprint or in other bioinformaticprocedures to identify characteristics of the sample, such as tissuetype or organismal identity.

In addition, as each polypeptide affixed to the substrate is optionallymonitored individually, the method is useful for the quantitativeanalysis of protein expression. For example, in some embodiments, themethod comprises comparing the sequences of each polypeptide, groupingsimilar polypeptide sequences and counting the number of instances ofeach similar polypeptide sequence. The methods described herein aretherefore useful for molecular counting or for quantifying the number ofpolypeptides in a sample or specific kinds of polypeptides in a sample.

In a further embodiment, cross-linked polypeptides or proteins aresequenced using the methods described herein. For example, across-linked protein may be affixed to a substrate and two or moreN-terminal amino acids are then probed and sequenced. The overlappingsignals that are detected correspond to probes each binding the two ormore N-terminal amino acids at that location. In one embodiment, it ispossible to deduce or deconvolute the two multiplexed/mixed sequencesvia a computational algorithm and DB search.

In a further embodiment, the methods described herein are useful for theanalysis and sequencing of phosphopeptides. For example, polypeptides ina sample comprising phosphopeptides are affixed to the surface of thesubstrate via metal-chelate chemistry. The phosphopolypeptides are thensequenced according to the methods described herein, thereby providingsequence and quantitative information on the phosphoproteome.

The following examples illustrate embodiments of the invention and arenot intended to limit the scope of the invention.

Example 1 Affinity Capture Reagents Based on the CIpS Adaptor ProteinScaffold

Phage display and combinatorial site-directed mutagenesis is used toidentify variants of the natural N-terminal amino acid binding pocket orstructural domain of the CIpS adaptor protein family that selectivelybind N-terminal amino acids. Structural modeling of the binding pocketand protein engineering are used to further define modified variants ofCIpS family members that are suitable for use with the methods describedherein.

Results

Modified CIpS adaptor proteins are selected using screening proceduresfamiliar to those trained in the art. Affinity capture reagents aresubsequently identified that exhibit high affinity/selectivity by phagedisplay to N-terminal amino acids.

Example 2 Single Molecule Sequencing of a Synthetic Polypeptide

An artificial test polypeptide comprising of the heptapeptide amino acidsequence TyrPheArgTyrPheArgLys (SEQ ID NO: 5) is synthesized. Thepolypeptide is affixed to a substrate via its C-terminal amino acidcarboxy group or the lysine side chain. The substrate is then washed inorder to remove any debris. Probes containing an N-terminal affinitycapture reagent identified as shown in Example 1 are coupled to afluorescent moiety. The probes are then added to the substrate underconditions that encourage the binding of the probes to polypeptide. Thesubstrate is then washed to remove any non-specifically bound probes.The probe bound to a single affixed polypeptide is then detected usingan optical detection system. The identity of the probe bound to thepolypeptide is recorded and the N-terminal amino acid of the polypeptideis cleaved via Edman degradation. Additional rounds of probes are thenadded to the substrate in order to detect and record the next N-terminalamino acid in the polypeptide prior to cleaving the N-terminal aminoacid.

Results

The sequential detection of the probes during each round of sequencingcorresponds to the sequence of the artificial polypeptide from theN-terminus to the C-terminus TyrPheArgTyrPheArgLys (SEQ ID NO: 5).

Example 3 Validation of Assay Conditions and Protocols

Variants of CIpS are derived via structural modeling, docking,combinatorial site-directed mutagenesis, and/or the experimentalselection of high affinity and high specificity binders by phage displayas shown in Example 1.

Recombinant CIpS protein is prepared for use as a probe by expression inE. coli, or other expression system, and purified using standardbiochemical methods, and subsequently coupled with one or more quantumdots with defined absorbance and emission wavelengths, including nearinfrared fluorescence emitters. The labels can be coupled to anN-terminal region of CIpS that is distinct from the C-terminal domainthat serves as the actual peptide ligand (i.e. N-terminal amino acid)binding pocket.

A glass or polystyrene substrate is coated with PEG/NHS, or equivalentreactive carbohydrate linker, to minimize non-specific adsorption andspurious background signal.

The test proteins and peptides include a panel of synthetic peptides ofknown sequence, some with confirmed binding to CIpS and others withpermuted N-terminal residues that aren't recognized by natural forms ofCIpS, as reported in the literature, as well as common standard proteinslike bovine serum albumin (BSA) and proteolytic digests thereof. Aseries of test sample dilutions using known amounts and numbers ofmolecules of the peptides/proteins at specific (serial fold)concentrations are generated prior to peptide coupling.

The test proteins/peptides are then affixed to the substrate, which iswashed, and repeatedly probed to assess target detection specificity andaffinity, detection limits, non-specific background signal (e.g. probeadsorption) and the response linearity based on molecular counting as aquantitative readout. Quantum dot signals are recorded with a suitable(i.e. CCD-enabled) optical microscope, and specific probe bindingsignals deconvoluted using suitable filters and software.

Results

The results show single-molecule detection of the test polypeptidesequences. The system exhibits low background and is able toconsistently and accurately sequence the test polypeptides.

Example 4 In Vitro Screens Using Phage Display to Identify N-TerminalAmino Acid Probes

The present inventors have engineered protein domains that recognizeN-terminal residues of polypeptides and are able to discriminate betweendifferent residues. Such protein domains are useful for the polypeptidesequencing methods as described herein and also constitute a valuableresource for other applications. The bacterial protein CIpS contains adomain that preferentially binds leucine, phenylalanine, tyrosine andtryptophan N-termini with single-digit micromolar to nanomolaraffinities, whereupon it mediates the transfer of these substrates tothe CIpAP protease for degradation (Schuenemann et al. (2009) Structuralbasis of N-end rule substrate recognition in Escherichia coli by theCIpAP adaptor protein CIpS. EMBO Reports 10:508-514 2009; Tasaki et al.,(2009) The substrate recognition domains of the N-end rule pathway. JBiol Chem 284: 1884-1895 2009).

The CIpS binding domain coordinates the alpha-amino group with a networkof hydrogen bonds while the side chain specificity in CIpS results frominsertion of the first side chain into a hydrophobic pocket.

In order to investigate and identify N-terminal amino acid probes basedon the CIpS protein, a truncated wildtype CIpS polypeptide comprising 84amino acids of wildtype E. coli CIpS (See UNIPROT Accession No. POA8Q6)was used as follows:

(SEQ ID NO: 1) 1          11         21         31         KPPSMYKVIL VNDDYTPMEF VIDVLQKFFS YDVERATQLM 41         51         61         71         81LAVHYQGKAI CGVFTAEVAE TKVAMVNKYA RENEHPLLCT LEKA

However, the natural CIpS binding domain has only a limited repertoireof specificities, with only low to moderate selectivity. Using modelingand directed evolution approaches, new domains or “variants” startingwith the natural CIpS domain as a scaffold have been generated withdifferent specificities and improved selectivity.

Large custom combinatorial libraries of variants surface displayed onbacteriophage (i.e. phage display) were generated on the basis ofavailable structural information generated by x-ray crystallographicanalysis of the ligand binding interfaces. In vitro selection(‘panning’) was then used to recover subsets of variants with desirablebinding properties as shown in FIG. 5. The in vitro selection conditionswere manipulated to control the selective pressure for the recovery ofhighly discriminating binders.

N-terminal affinity capture probes were thereby generated that exhibit(i) highly selective peptide binding (i.e. single N-terminal residuespecificities), and (ii) have novel binding capabilities not seen withthe wildtype CIpS scaffold).

Phage Display

CIpS domains from two different bacterial species (the gram negatives E.coli and Caulobacter crescentus) were tested to demonstrate that theCIpS domains displayed on phage as N-terminal fusions to a phage surfaceprotein were functional, i.e. able to bind their cognate ligands. TheCIpS domains were N-terminally FLAG-tagged as shown in FIG. 6A so thatdisplay of the recombinant phage coat protein would be detectable (usingcommercial anti-FLAG antibody), even if the domains were not able tobind ligand. Several linkers were also tested for the presentation ofsuitable synthetic peptide ligands to find sequences that supporteddomain binding without exhibiting undue non-specific binding (data notshown). The expressed proteins also contained an ST2 secretion signalthat is cleaved off and a C-terminal phage coat protein p3 (that is,fused to the C-terminus/residue 106 of E. coli CIpS) as shown in FIG.6C. The WT CIpS expression protein used for Phage-displayed CIpS fromEscherichia coli (E. coil) was able to bind its cognate ligand(immobilized Leucine) as shown in FIG. 6B, indicating that the CIpSdomain was functionally displayed and hence suitable for use as ascaffold.

Library Construction

Combinatorial libraries of CIpS domain variants were generated usingoligo-directed mutagenesis to introduce amino acid diversity into keypositions (i.e. putative ligand determinants).

The CIpS libraries were used to gather extensive data on the sort ofamino acid diversity that would be structurally tolerated at keypositions forming (i.e. near or in) the ligand binding pocket with thegoal of generating a highly functional library that exploits as muchamino acid diversity as possible, while minimizing the number of libraryvariants that destabilize or result in an unfolded scaffold.

Given that there are two components to N-terminal ligand recognition(specificity pocket residues, and alpha-amino recognition), twolibraries were designed as shown in FIG. 7. In the first library, thespecificity pocket residues were hard randomized to generate variantswith all possible genetically encoded amino acids at the designatedresidues shown in FIG. 7A (corresponding to residue positions 17, 18,21, 40, 43, 76 and 77 of the WT truncated CIpS polypeptide (SEQ IDNO: 1) shown as positions 39, 40, 43, 62, 65, 98 and 99 in FIG. 7A). Inthe second library, the residues responsible for alpha-amino recognitionwere hard randomized as shown in FIG. 7B (corresponding to residuepositions 12, 13, 14, 16, 17, 44, and 76 of the WT truncated CIpSpolypeptide (SEQ ID NO: 1) shown as positions 34, 35, 36, 38, 39, 66 and98 in FIG. 7B).

Target Selection

The N-terminal FLAG tag was used to select for protease resistantdomains expressed on the phage surface using an immobilized anti-FLAGantibody. Only domains that are reasonably well folded will escapedegradation by host bacterial proteases during the phage amplificationprocess. These domain were recovered and a large number were sequencedin order to determine a sense of the amino acid diversity that isstructurally tolerated at each randomized position. The results of thesestructure selections indicate that the specificity pocket residues ofCIpS are surprisingly tolerant to the full range of amino aciddiversity.

For example, as seen in FIG. 8A, while the most frequently observedresidue at position 99 is the same as the wildtype (Leu), followed byother aliphatic residues, polar and charged residues are also commonlyobserved at that position. Likewise, the alpha-amino recognitionpositions shown in FIG. 8B are also very tolerant for residuealterations.

Given the discovery that the specificity pocket library was functionallyrobust, selections against synthetic peptide ligands were carried outusing this library. The specificity pocket library was selected against20 biotinylated peptides, each with a different N-terminal residue atopa common leader sequence (X-SDGMFTAGSLIGK(biotin)) (SEQ ID NO: 4). Thepeptides were immobilized individually to the bottom wells of amicrotiter plate. In order to recover highly selective binders,competitors were included in solution, i.e., non-biotinylated peptideswith different N-terminal residues, during each round of panning againstthe immobilized ligands. The most stringent competition employed was toinclude eighteen peptides with the other N-terminal residues (excludingthe immobilized residue and cysteine) at a concentration slightly higherthan the reported dissociation constant of the wildtype ClpS:Leu-peptideinteraction (Kd=4.8 uM; competitor concentration=10 μM). Consequently,CIpS mutant variants that prefer other residues in addition to theligand ‘bait’ would bind those competing peptides in solution and simplybe washed away. Only selective variants that bound to the immobilizedtarget(s) were amplified during each subsequent round of selection.

In addition to the pooled competitor selection, two less stringentcompetition strategies were also performed: a) Leu- and Phe-peptides(both at 10 μM), in order to recover variants that prefer anything otherthan the wildtype ligands; and Gly-peptide (at 10 μM), in order torecover domains that have higher affinity for any N-terminus side chainrather than for no side chain. Additionally, selections without anycompetition were carried out, as a baseline for evaluating theeffectiveness of the competition strategies and concentrations.

To evaluate the specificity and selectivity of individual clones, bothwild-type CIpS and the selected variants' binding to each of thebiotinylated peptides independently were tested. The same approach wasalso used to monitor the progress of the selections and to prioritizeparticular pools of phage for further investigation. The resultingspecificity profile of the phage displaying wildtype CIpS is shown inTable 1. As expected, CIpS showed a clear preference for Leu, Tyr, Pheand, more weakly, Trp peptides.

TABLE 1 Phage displayed wild-type E. coli CIpS shows binding preferencesfor a panel of immobilized peptides consistent with publishedbiophysical data and peptide profiling. N-terminal amino acid CIpS Wt A−0.29 C −0.23 D −0.29 E −0.26 F 1.81 G −0.04 H −0.03 I −0.016 K 0.05 M−0.11 N −0.05 P −0.07 Q −0.18 R 0.13 S −0.01 T 0.01 V −0.09 W 0.77 Y1.08 L 1.57 N/A −0.29 N/A −0.11

Promising single clones were picked from the selection pools and theirspecificity was evaluated using the same panel of biotinylated peptidesand various amounts of phage supernatant. As shown in Table 2, probevariants exhibiting markedly different binding properties compared tothe native CIpS protein were generated. In particular, CIpS Variant 1bound specifically to tryptophan alone, a level of specificity which hasnot been reported before, and also gave a much stronger signal thanwildtype CIpS, indicating it is more efficiently displayed and/or morestable than the wildtype sequence. Accordingly, Variant 1 represents aselective N-terminal amino acid capture probe for tryptophan useful forthe sequencing methods described herein. Variant 1 also represents auseful scaffold for performing further generation and selection ofvariants with desirable properties for use as additional N-terminalamino acid capture probes.

In addition, Variant 2 was shown to have good selectivity for lysine.Variants 3 and 4 preferentially recognized glutamine and aspartic acidrespectively, amino acids that are not recognized by wildtype CIpS.Accordingly, probes that exhibit (i) highly selective ligand binding(single N-terminal residue specificities), and (ii) different bindingcapabilities as compared to the wildtype CIpS scaffold can readily begenerated using the methods described herein.

TABLE 2 Engineered CIpS variants shows differential binding to peptideligand N-termini. CIpS WT Variant 1 Variant 2 Variant 3 Variant 4 10X 1X5X 1X 1X A −0.29 0.018 −0.049 −0.015 0.009 C −0.23 0.066 −0.059 −0.0050.021 D −0.29 −0.017 −0.07 −0.003 0.91 E −0.26 0.032 −0.087 −0.023 0.033F 1.81 0.083 −0.207 −0.006 0.035 G −0.04 0.028 −0.135 −0.006 0.011 H−0.03 −0.001 −0.113 −0.031 0 I −0.16 −0.023 −0.079 −0.027 −0.022 K 0.050.129 −0.201 −0.007 0.01 M −0.11 −0.003 −0.072 −0.017 −0.003 N −0.050.052 −0.063 −0.011 0.002 P −0.07 0.062 0.153 −0.039 −0.022 Q −0.180.065 −0.178 0.356 −0.016 R 0.13 0.04 0.429 −0.082 −0.051 S −0.01 −0.001−0.108 0.022 0.013 T 0.01 −0.005 −0.033 −0.042 −0.029 V −0.09 0.032−0.234 −0.002 −0.006 W 0.77 2.636 −0.223 −0.019 −0.017 Y 1.08 0.045−0.235 0.006 −0.001 L 1.57 0.069 1.609 −0.031 −0.006 NA −0.29 0.064−0.015 0.025 0.031 NA −0.11 −0.023 −0.052 0.014 0.016 FLAG — — — — — subFab sub Fab sub Fab sub Fab sub Fab

Materials and Experimental Methods

Phage screens

The selection and ELISA protocols are very similar. Briefly, each wellof a 96-well Maxisorp plate was coated overnight at 4° C. with 100 μlof10 μg/ml neutravidin in PBS, or 1:500 dilution of anti-FLAG antibody.Plates were washed three times with PBS+0.50% TWEEN-20 (PT) and blockedfor 1 hour at room temperature with 200 ul of PBS+0.5% BSA (PB). Plateswere washed again three times with PT. Stock solutions of biotinylatedpeptides (4mg/m1 in 100% DMSO) were diluted to 500 pM in PBS+5% DMSO.Dilute peptide solution was incubated on neutravidin-coated wells for 1hour at room temperature, and then plates were washed three times withPT.

For selections, 100 ul of PEG-precipitated library phage, or pH adjustedphage supernatant from overnight culture, was added to appropriate wellsand allowed to bind for 2 hours at 4° C. The plate was then washed sixtimes with cold PT. Phage were eluted by addition of 100 ul of log-phaseXL1Blue culture OD600=0.4) directly to the selection plate andincubation for 30 minutes at 37C with 200 rpm. Helper phage (M13K07) wasthen added to a final concentration of 1010 phage/ml and the selectionplate was incubated for another 45 minutes at 37C with 200 rpm. Finally,the cells were transferred to 1.3 ml of 2YT+carbenicillin (100pg/ml)+kanamycin (25 pg/ml) in a deep well plate and grown overnight at37° C. with 200 rpm. The following day, the cells were precipitated by10 minute centrifugation at 3000xg at 4° C. The phage supernatant wasthen transferred to a clean deep well plate and the pH adjusted byaddition of 10X PBS to a final concentration of 1X.

Enzyme-Linked Immunoassays

For the ELISA readouts, 100 μl of pH adjusted phage supernatant orpolyethylene glycol (PEG) precipitated concentrated phage was added toappropriate wells and allowed to bind for 1 hour at 4° C. The plate wasthen washed three times with cold PT. anti-M13:HRP conjugated antibodywas diluted 1:3000 in cold blocking buffer and 100 μl aliquotted to eachwell, for incubation at 4° C. for 15 minutes. The plate was then washedsix times with cold PT. 90 μl of TMB detection substrate was added toeach well and allowed to develop for approximately 5 minutes. Thereaction was stopped by addition of 90 μl of 1M H3PO4 and the absorbanceat A450nm was read.

Example 5 CIpS Probe Labeling and Detection

In order to allow for the detection of N-terminal amino acid affinitycapture probes bound to individual surface-immobilized polypeptides, theprobes are optionally detectably labeled with a fluorescence-basedmarker. Accordingly, various methods of labeling the probes wereinvestigated using FluoSpheres™ (FS) (Invitrogen), which comprisepolystyrene shells encompassing core dye molecules. FluoSpheres™ withthe same surface chemistry are available in a useful range of sizes(20nm-1 μm diameter). In addition, FluoSpheres with a number ofdifferent dyes with varying emission spectra to generate discernibleprobes for multiplex analysis are available.

20 nm diameter FluoSpheres (FS20) with the Nile Red dye were selected tominimize the footprint of the peptide-bound labeled CIpS. However, ifthe fluorescence intensity is found to be inadequate for asingle-molecule detection, a larger particle can also be used.

Two different (indirect and direct) conjugation schemes were used toattach fluorescent labels to CIpS proteins as shown in FIGS. 9 and 10.The first (indirect) labeling approach is based on the fact that therecombinant CIpS used for the present Experiments was expressed andisolated from E. coli as fusions with an N-terminal glutathionetransferase (GST) domain, which strongly binds glutathione (GSH).Therefore, after the FluoSpheres are functionalized with GSH, mixingFS20 with the purified protein should effectively fluorescently labelthe CIpS probe through the GST-GSH interaction. Furthermore, since thisconjugation approach involves an interaction with the GST rather thanthe amino acid-binding domain of CIpS, it is not expected tosignificantly interfere with the peptide-binding functionality of theprobes.

Indirect Probe Labeling

Glutathione is a tripeptide of glutamine, cysteine and glycine, withcysteine in the central position. To prevent GSH immobilization frominterfering with GST binding, GSH was attached to the FluoSpheresthrough thiol of the cystein residue using a long linker molecule (seeChen et al. Effect of Linker for Immobilization of Glutathione on BSAAssembled Controlled Pore Glass Beads. Bull. Kor. Chem. Soc. 25 (2004)1366-1370). Sulfo-LC-SPDP was selected as a suitable ˜1.5 nm longheterofunctional linker molecule with thiol and amine-reactive ends. Inorder for this molecule to be used for GSH immobilization on FS20, theFluosphere surface was first functionalized with amines. This wasachieved by reacting FS20 with ethylenediamine in a process catalyzed bythe 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) crosslinker,which forms an amide bond between the carboxylic acids and amines.Amine-functionalized FS20 (diaminated FS20-DA) was then reacted withSulfo-LC-SPDP, leaving the thiol reactive end for further conjugation tothe GSH. Once the FluoSpheres containing different dyes werefunctionalized with GSH, they were used to fluorescently labelend-terminal amino acid binding CIpS proteins simply by mixing the twopopulations together. Probes directed to different N-terminal aminoacids can also readily be labeled with a differentially colored dye,allowing for the use of multiplexing in probe detection.

Direct Probe Labeling

The second (direct) labeling approach involves direct covalentattachment of the fluorescent label to the probe. This is achieved byusing EDC to form an amide bond between the carboxylic acids on FS20surface, and the primary amines of the CIpS probe. This scheme issimpler than the first one, since it does not require Fluosphere™functionalization with glutathione, however, the drawback is that thelocation of the reacting amine on the CIpS protein cannot be controlled.If the attachment preferentially takes place on the ClpS peptide-bindingdomain rather than the GST moiety, it might interfere with the primaryfunction of CIpS protein to bind a terminal amino acid on a peptide.

Making Amine-Functionalized Fluo-Spheres

20 nm diameter FluoSpheres™ (FS20), purchased from Invitrogen, containedcarboxylic acid functional groups on their surface. The synthetic schemefor adsorbing glutathione (GSH) onto their surface required the presenceof amine groups instead. Amine functionalization of FS20 (FS20-DA) wasperformed by mixing FS20 and ethylene diamine (DA) in the presence of1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) crosslinker, whichcatalyzes the formation of a stable amide bond between a carboxylic acidand an amine.

The resulting FluoSpheres were evaluated by agarose gel electrophoresisby fluorescence imaging of the gel (data not shown). FS20 functionalizedwith carboxylic acids have a negative (−) surface charge, and thereforemigrate in the direction of the cathode (+). When functionalized withamines, FS20-DA surface is expected to attain a more positive charge(negative carboxylic acids are replaced with positive amines). Thisshould lead to a slower gel migration of FS20-DA than of FS20 in the (+)direction (assuming only a fraction of carboxyl groups get modified, sothat overall surface charge is still negative; if majority or carboxylicacids are modified the net surface charge will be positive, leading tomigration in the (−) direction). In agreement with these predictions,for FS20 reacted with dilution series of ethylene diamines, as theconcentration of the diamine was increased, a larger number of carboxylgroups get replaced with amines, leading to a slower migration of theFluoSpheres™ on the gel. The results further demonstrated that only aportion of carboxylic acids reacts with diamines: even in the conditionsof diamine saturation, FS20-DA still migrate in the direction ofcathode, indicating net negative surface charge remains.

Amine-functionalization of FS20 was further confirmed by reactingFS20-DA with a FITC dye that reacts and forms bonds with amine groups(Fluorescein isothiocyanate isomer I). The expectation was that as theFS20-DA amine density increases when generated using higherconcentrations of diamine, the quantities of dye associated with FS20-DAshould increase as well. Since FITC emission peaks at a differentwavelength than Nile Red, the presence of FITC dye could be detected bylooking at the spectral emission profile of the samples. Excitation ofFITC at 495 nm also excites Nile Red, so that normalization of FITCemission to the Nile Red emission peak accounts of the inter-samplevariation of Fluosphere™ concentration. This allows for a quantitativecomparison of FS20-DA surface-immobilized FITC dye. The results (datanot shown) showed normalized FITC emission for the dye reacted with theFS20-DA dilution series samples. Moreover, the magnitude of FITCemission increased with higher concentrations of diamine reacted withFluoSpheres in a manner similar to the gel shifts.

Reacting Sulfo-LC-SPDP with the Amines on Fluosphere Surface

FS samples (1 M, 100 mM, and 10 mM) were reacted with Sulfo-LC-SPDP(referred to as SPDP) to form SPDP-functionalized FluoSpheres™(FS20-SPDP). Following the reactions, FS20-SPDP were visualized usingagarose gel electrophoresis. SPDP conjugation to amines on the surfaceof FluoSpheres blocks the (+) charge associated with the amines (SPDP isa neutral molecule and does not contribute to the charge). This leads toa more negative net surface charge, resulting in faster gel migration(effectively negating some of the charge difference between FS20 andFS20-DA). The SPDP-functionalized fluosphere samples were further probedwith the amine-reactive FITC dye described above. Since some of thesurface amine groups react with SPDP, FS20-SPDP should have fewer aminegroups on their surface than FS20-DA. As a result, FITC dye associationwith FS20-SPDP should be lower as well. Indeed, this was the observedtrend: after reaction with SPDP the different FS20-DA samples showedlower FITC dye binding (data not shown).

Conjugating SPDP-Functionalized FS20 to Glutathione (GSH)

In a new set of reactions, FS20 was sequentially reacted with ethylenediamine and SPDP. FS20-SPDP were then further conjugated to GSH(FS20-GSH). All of the samples were visualized on the agarose gel. Asobserved previously, amination of FluoSpheres leads to a slowermigration of the particles on the gel. Introduction of SPDP groupsblocks some of positive amine charges, and some of the fluospheremobility is restored. Since GSH is negatively charged, FS20-GSH havehigher net negative surface charge, and migrate faster towards thecathode than FS20-SPDP. Taken together, the data demonstrates thestepwise functionalization of carboxy-functionalized FS20 withglutathione.

Labeling CIpS Proteins with FS20 Through EDC-Catalyzed Reaction

CIpS proteins were conjugated directly to carboxyl-functionalized FS20using EDC to crosslink carboxylic acids on FS20 with amines on CIpS.Three different concentrations of CIpS were used (2.8, 28, and 280CIpS-to-FS20 ratio). In addition, controls without EDC were included toestimate non-specific binding between CIpS and F520. All of the sampleswere visualized by gel electrophoresis: robust EDC-catalyzedcrosslinking was observed, and the number of CIpS per FS20 (representedby the magnitude of the band shift) increased with increasingCIpS-to-FS20 ratio. Some residual adsorption of CIpS onto FS20 wasobserved in the absence of EDC crosslinker, but the EDC-based covalentassociation was much stronger.

These results demonstrate that recombinant CIpS can be fluorescentlylabeled directly with carboxyl-functionalized FluoSpheres in a simpleone-step process.

Labeling of CIpS with FS20-GSH Through GSH-GST Interaction

FS20-GSH was incubated with the GST-tagged recombinant CIpS protein witha ClpS/FS20-GSH ratio of 250. The expectation was that the GSHimmobilized on FluoSpheres would interact with the GST domains of CIpS,leading to the association between the proteins and FS20 molecules. Theinteraction between FS20-GSH and CIpS was probed using agarose gelelectrophoresis followed by fluorescent imaging (data not shown). Whilethe association between FS20-GSH and CIpS does take place, the datasuggest that the degree of association is lower than for theEDC-catalyzed reaction.

Binding of Labeled CIpS Protein to Surface-Immobilized Peptides

Arg- and Leu-terminated biotinylated peptides were immobilized inNeutrAvidin-coated wells of a 96-well plate through biotin-NeutrAvidininteractions. Wells containing NeutrAvidin alone but no peptides wereincluded as controls to estimate non-specific adsorption. Fluorescentlylabeled CIpS protein was then added to the wells and incubated to allowfor the protein-peptide binding to occur. To exclude the possibilitythis might be due to non-specific binding, the wells were incubated withbound FS20-GSH-Clps in buffer containing reduced GSH. GSH present inexcess was expected to compete for binding to the GST domain ofrecombinant proteins, thus releasing FS20-GSH into solution (observingfluorescence in solution should lead to lower background signal, sincein the case of immobilized peptide the excitation laser focuses on thebottom of the well, thus maximizing the plate plastic'sautofluorescence).

As shown in FIG. 12, CIpS labeled by FluoSpheres™ both through covalentconjugation or GST-GSH based interaction showed preferential binding forthe Leu-terminated peptide. The magnitude of this interaction appearedstronger for the covalently labeled protein, which is consistent withobserved lower CIpS-FluoSphere association with the GST-GSH labelingmethod.

Accordingly, CIpS proteins can be detectably labeled and used toidentify the N-terminal residues on immobilized peptides. The results ofthese experiments, combined with the agarose gel data that indicatesstable FluoSphere-protein association, demonstrate the advantages of thetwo labeling methods investigated in this pilot study: (i) directEDC-catalyzed covalent conjugation leads to higher protein-to-FS20loading ratios, which translates to stronger association (and hencesignal) between FS20-CIpS and (plate) immobilized peptides; and (ii)indirect labeling through GST-GSH interaction.

Materials and Experimental Methods Imaging Materials and Equipment

20 nm Nile Red (535/575) FluoSpheres™ were purchased from InvitrogenCanada Inc. (Burlington, ON). All other reagents were purchased fromSigma-Aldrich Canada Ltd. (Oakville, ON), except for Sulfo-LC-SPDP(X-Link Bioscience Inc., Freeport, Ill.). NAP-5 Sephadex desaltingcolumns were purchased from GE Healthcare Life Sciences (Baie d′Urfe,QC). All agarose gels (1%) were run at 50 V. Fluorescence spectralmeasurements were performed using FluoroMax-3 spectrofluorometer (HORIBAInstruments Inc., Ann Arbor, Mich.).

Functionalizing FluoSpheres with Glutathione (GSH)

All of the reactions were performed in 0.1 M sodium phosphate bufferwith 0.05% TWEEN (SPBT). The buffer was kept at pH 6.8 for EDC-basedreactions, and adjusted to pH 7.2 for the SPDP conjugation. To makeamine-functionalized FluoSpheres (FS20-DA), 100 μL of 20 nm Nile RedFluoSpheres (FS20) (2 mg/mL in SPBT) were mixed with 100 μL of 100 mMethylenediamine dihidrochloride solution (in SPBT), then EDC (40 mg/mLin H2O, prepared immediately before use) was added to a finalconcentration of 2 mg/mL. The samples were incubated for 2 hours at roomtemperature, then purified by 2 rounds of desalting using NAP-5 SEPHADEXcolumn. When generating FS20-DA using dilution series of diamine (1 M,100 mM, 10 mM, 1 mM, 100 μM, or 10 μM, all in SPBT), 8 rounds ofultracentrifugation (300,000×g, 1 hour each) were used for purification.

To make SPDP-functionalized FS20 (FS20-SPDP), FS20-DA solution wasadjusted to pH 7.2 by addition of NaOH, then 50 μL of Sulfo-LC-SPDP (5.2mg/mL in H2O, prepared immediately before use) was added per 1 mL ofFS20-DA suspension. The samples were incubated for 2 hours and purifiedby one round of desalting. When FS20-SPDP were generated from thediamine dilution series FS20-DA, 8 rounds of ultracentrifugation(300,000×g, 1 hour each) were used for purification.

To make GSH-functionalized FS20 (FS20-GSH), 50 μL of reducedL-Glutathione suspension in H2O(5.4 mg/mL, prepared immediately beforeuse) was added per 1 mL of FS20-SPDP solution. The samples wereincubated overnight, then purified by 3 rounds of desalting (to removeany remaining unbound GSH, Sulfo-LC-SPDP, diamine).

Reacting Amine and Sulfo-LC-SPDP Functionalized FS20 with Amine-ReactiveFluorescent FITC Dye

10 μL of 1 mM Fluorescein isothiocyanate isomer I dye solution(suspension in H2O was added to 90 μL of FS20-DA or FS20-SPDP. Thesamples were incubated for 2 hours, and purified by 3 rounds ofultracentrifugation (300,000×g, 1 hour each) in the case of FS20-DA, and5 rounds of purification for FS20-SPDP.

Labeling CIPS Protein Through GSH-GST Interaction

The labeling was achieved simply by mixing the suspensions of FS20-GSHand CIpS protein. No purification step was performed.

Labeling CIpS by EDC-Catalyzed Covalent Crosslinking

Equal volumes of the protein suspension (CIpS) and stockcarboxylfunctionalized FS20 were mixed together. EDC (40 mg/mL in H2O,prepared immediately before use) was added to a final concentration of 2mg/mL. The samples were incubated for 2 hours. No purification step wasperformed.

Binding of Labeled CIpS protein to Surface-Immobilized Peptides

NeutrAvidin and anti-GST antibodies were immobilized in the wells of96-well Nunc-Immuno™ Plates (100 μL of 10 pg/mL solution incubated for 2hours at room temperature). Peptides (arginine and leucine terminated)conjugated to biotin were further adsorbed onto the NeutrAvidi-coatedsurface through biotin-NeutrAvidin interaction (50 pM per well). LabeledFS20-CIpS and FS20-GSH-CIpS were added at 10 particles/mL in SPBT bufferand incubated at 4° C. for 1 hour. The wells were then washed 6 timeswith SPBT, and fluorescence measurements were taken (dry wellmeasurements). SPBT buffer with 10 mM GSH was then added to wells withF520-GSH-CIpS, and incubated for 1 hour to release FS20-GSH intosolution, following which the fluorescence measurements were taken.Fluorescent measurements were made using PHERAstar fluorescence platereader (BMG Labtech, Offenburg, Germany).

Example 6 Characterization of the CIpS Variant 1 Trp-Binding N-terminalAmino Acid Probe

The Trp-binding CIpS Variant 1 identified in Example 4 was isolated andsequenced in order to further characterize the properties of theN-terminal amino acid probe. The sequence of Variant 1 was determined tobe as follows:

(SEQ ID NO: 2) KPPSMYKVILVNDDYTPAEFCIDVLQKFFSYDVERATQLCLAAHYQGKAICGVFTAEVAETKVAMVNKYARENEHAALCTLEKA

Compared to the wildtype CIpS sequence, the Trp-binding CIpS Variant hadmutations at positions 18, 21, 40, 43, 76 and 77 as shown in SEQ ID NO:2. It is noted that the cystein variants at positions 21 and 40 may forma disulfide bond, which serves to stabilize the variant polypeptidewhile still allowing for N-terminal amino acid binding. FIG. 12 showsthe sequence of wildtype CIpS (delta 1-34) as well as the Trp bindingCIpS variant with key residues highlighted.

Competitive phage was also used to investigate the Trp binding CIpSvariant and demonstrate that only Wpep is an effective competitor forbinding. Phage bearing CIpS variant W1 were incubated with the indicatedconcentration of non-biotinylated peptide (X=N-terminal residue asindicated in FIG. 12, XSDGMFTAGSLI) and binding was allowed to reachequilibrium. Each phage pool was then briefly incubated in wells of amicrotitre plate with Wpep (ASDGMFTAGSLIGK(biotin)) immobilized onneutravidin. Non-binding phage were washed away, and retained phage weredetected using an enzymatically-labeled antibody against the filamentousphage particle and an appropriate colorimetric substrate. The value forW1 binding to neutravidin alone was subtracted from the binding signalfrom every well to correct for non-specific binding. The ELISA valueswere then expressed as proportions, relative to the maximum ELISA signalin absence of competitor. Error bars represent 95% confidence intervals(n=2). As shown in FIG. 12, the concentration of Wpep competitorrequired to block 50% of variant W1 binding (inhibitory concentration 50or IC50) is approximately 1 uM. Wpep competitor at 10 uM reduces bindingto 11%. At that same concentration, peptides with other N-terminalresidues are much less effective competitors (F, Y, I, G, L) orineffective (all other natural residues), demonstrating CIpS variantW1's binding selectivity.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosures as come within known or customary practice withinthe art to which the invention pertains and as may be applied to theessential features herein before set forth, and as follows in the scopeof the appended claims.

All publications, patents and patent applications are hereinincorporated by reference in their entirety to the same extent as ifeach individual publication, patent or patent application wasspecifically and individually indicated to be incorporated by referencein its entirety.

1. A method of sequencing a polypeptide comprising: a. affixing thepolypeptide to a substrate; b. contacting the polypeptide with aplurality of probes, wherein each probe selectively binds to anN-terminal amino acid or a N-terminal amino acid derivative; c.detecting the probe bound to the polypeptide molecule, therebyidentifying the N-terminal amino acid of the polypeptide; d. cleavingthe N-terminal amino acid or N-terminal amino acid derivative of thepolypeptide; and e. repeating steps (b) to (d) to determine the sequenceof at least a portion of the polypeptide.
 2. The method of claim 1,wherein the polypeptide is a single polypeptide molecule.
 3. The methodof claim 1, wherein step b) further comprises derivatizing theN-terminal amino acid of the polypeptide prior to contacting thepolypeptide with the plurality of probes.
 4. The method of claim 1,wherein step a) comprises affixing the polypeptide to the substratethrough a C′-terminal carboxyl group or a side chain functional group ofthe polypeptide.
 5. The method of claim 1, wherein the polypeptide iscovalently affixed to the substrate.
 6. (canceled)
 7. The method ofclaim 1, wherein the substrate comprises a plurality of spatiallyresolved attachment points.
 8. The method of claim 7, wherein step a)comprises affixing the polypeptide to a spatially resolved attachmentpoint.
 9. The method of claim 1, wherein the plurality of probescomprises: a. one or more probes that selectively bind to one of 20natural proteinogenic amino acids; b. one or more probes thatselectively bind to a post-translationally modified amino acid; or c.one or more probes that selectively bind to a derivative of a) or b).10. The method of claim 11, wherein at least one probe comprises anaffinity capture reagent and a detectable label.
 11. The method of claim10, wherein the step of detecting the probe bound to the polypeptidecomprises detecting the detectable label
 12. The method of claim 10,wherein the affinity capture reagent comprises a polypeptide. 13.(canceled)
 14. (canceled)
 15. The method of claim 1, wherein step e)comprises cleaving the N-terminal amino acid or N-terminal amino acidderivative of the polypeptide using Edman degradation.
 16. (canceled)17. A method of sequencing a plurality of polypeptide molecules in asample comprising: a. affixing the polypeptide molecules in the sampleto a plurality of spatially resolved attachment points on a substrate;b. contacting the polypeptides with a plurality of probes, wherein eachprobe selectively binds to an N-terminal amino acid or a N-terminalamino acid derivative; c. for a plurality of polypeptides molecule thatare spatially resolved and affixed to the substrate, identifying theprobe bound to each polypeptide; d. cleaving the N-terminal amino acidor N-terminal amino acid derivative of each of the polypeptides; and e.repeating steps b) to d) to determine the sequence of at least a portionof one or more of the plurality of polypeptide molecules that arespatially resolved and affixed to the substrate.
 18. The method of claim17, wherein the sample comprises a biological fluid, cell extract ortissue extract.
 19. (canceled)
 20. (canceled)
 21. A probe comprising avariant of a polypeptide wherein the variant binds to an N-terminalamino acid or a N-terminal amino acid derivative with a differentselectivity than the polypeptide.
 22. (canceled)
 23. The probe of claim21, wherein the polypeptide is a ClpS polypeptide and the variant of theClpS polypeptide comprises at least 80% sequence identity to SEQ ID NO:1 or SEQ ID NO:
 2. 24. The probe of claim 23, wherein the variantcomprises one or more mutations at positions that correspond to residues12, 13, 14, 16, 17, 18, 21, 40, 43, 44, 76 or 77 as set forth in SEQ IDNO:
 1. 25. The probe of claim 23, wherein the variant CIpS polypeptidecomprises cystein residues at positions that correspond to residues 21and 40 in SEQ ID NO:
 1. 26. The probe of claim 21, wherein the variantcomprises a polypeptide with at least 80% sequence identify to thepolypeptide sequence as set forth in SEQ ID NO: 2 and wherein thevariant selectively binds the N-terminal amino acid tryptophan.
 27. Theprobe of claim 21, further comprising a detectable label.